1. Field of the Invention
The present invention relates to a three-dimensional display unit capable of regenerating a stereoscopic image without requiring any special spectacles.
2. Description of the Related Art
In a general display technique, a three-dimensional display unit using a lenticular lens is known as a device for displaying a stereoscopic image without any spectacles. In particular, the three-dimensional display unit is realized in combination with a flat panel display such as a liquid crystal display since positions of the lenticular lens and display pixels are easily aligned with each other and a distance from a display face to the lenticular lens is short.
FIG. 1 shows one general example of the three-dimensional display unit of a direct viewing type in which the lenticular lens is directly stuck to the display face of a liquid crystal panel. The three-dimensional display unit shown in FIG. 1 is of a two-eye type as an example.
One portion of a parallax image for a left-hand eye is displayed in a pixel DD.sub.i1 of a liquid crystal panel 50. One portion of a parallax image for a right-hand eye is displayed in a pixel DD.sub.i2 of the liquid crystal panel 50. Index i is set to a value from 1 to n.
Stereoscopic signal sources 52 and 53 are respective sources of these parallax images. The parallax images are synthesized and displayed by a stereoscopic signal synthesizer 54. A lenticular lens 51 is arranged such that the lenticular lens 51 is closely attached onto a front face of the liquid crystal panel 50. A cylindrical lens LL.sub.i corresponds to a pair of pixels DD.sub.i1 and DD.sub.i2. Light is transmitted through the pixels DD.sub.i1 and DD.sub.i2 and is separated into light portions in display spaces C and D in an observation region by a converging action of the cylindrical lens LL.sub.i. An observer can observe a stereoscopic image when left-hand and right-hand eyes of the observer are respectively located in the spaces C and D.
In FIG. 1, the cylindrical lens LL.sub.i has the same lens shape, but a pitch of the pair of pixels DD.sub.i1 and DD.sub.i2 is different from that of the cylindrical lens LL.sub.i. In this case, the pitch of the cylindrical lens is set to be slightly smaller than the pitch of the pair of pixels DD.sub.i1 and DD.sub.i2. Accordingly, a center of the pair of pixels is shifted from that of the corresponding cylindrical lens in a peripheral portion of the liquid crystal panel. An amount of this shift is increased as this shift is caused in the peripheral portion. An incident angle of the transmitted light from each of the pixels to the cylindrical lens in a center of the liquid crystal panel 50 is different from that in the peripheral portion of the liquid crystal panel 50 by this shift. Accordingly, the transmitted light from pixels in the peripheral portion of the liquid crystal panel 50 can be converged into the specified spaces C and D in the observation region.
A three-dimensional display unit using a parallax barrier is known as a device of another display type capable of observing a stereoscopic image without any spectacles. FIG. 2 shows one general example of the three-dimensional display unit in which this parallax barrier is constructed by a liquid crystal panel.
The three-dimensional display unit shown in FIG. 2 is constructed by two liquid crystal panels each having the same performance and composed of a liquid crystal panel 61 for a display and a liquid crystal panel 62 for a slit barrier. The three-dimensional display unit shown in FIG. 2 is also constructed by a Fresnel lens 63 inserted between these liquid crystal panels 61 and 62, a personal computer 64 for generating a three-dimensional image and a slit barrier, etc. In this case, the two liquid crystal panels are laminated with each other in a direction in which polarizing directions of polarizing plates are in conformity with each other. Further, the two liquid crystal panels are arranged such that light from a back light arranged behind the liquid crystal panel 62 for a slit barrier is transmitted through the liquid crystal panels. The liquid crystal panel 62 for a slit barrier displays a slit image having a high contrast ratio. The liquid crystal panel 61 for a display displays multiple visual point images generated by computer graphics in a state in which the multiple visual point images are synthesized in a stripe shape. Thus, a stereoscopic image can be observed according to the principle of the parallax barrier as shown in FIG. 3. Namely, a parallax image 72 for a left-hand eye and a parallax image 73 for a right-hand eye are displayed in a stripe shape on a display panel 70. The left-hand eye can see only the parallax image for a left-hand eye and the right-hand eye can see only the parallax image for a right-hand eye when these parallax images are observed through a slit barrier 71 arranged on a front face of the display panel 70. Thus, a stereoscopic image can be observed.
At this time, a Fresnel lens 63 is used to set an opening pitch to be slightly smaller than an image pitch (see FIG. 2). This structure is similar to the above structure of a lenticular system in which the pitch of the cylindrical lens is slightly smaller than that of a display pixel.
The lenticular lens is fixedly arranged in the three-dimensional display unit of the lenticular system shown in FIG. 1. Therefore, for example, it is necessary to remake the lenticular lens when the two-eye type display is changed to a three-eye type display. Similarly, it is necessary to remake the lenticular lens when observation distances are changed. It is also necessary to remake the lenticular lens when a display panel having a different pixel pitch is used in the two-eye type display.
When a normal two-dimensional image is displayed in this general example, images having reduced resolutions are separately observed by left-hand and right-hand eyes. Accordingly, an observed image is different from an originally displayed two-dimensional image.
When a used display panel and an observation position are determined, the display unit is first simulated to design an optimum lenticular lens for a simulating condition. However, it is difficult to manufacture and attach the lenticular lens to the display unit as simulated since there are problems about a manufacturing technique of the lenticular lens at its manufacturing time, or mechanical problems in attachment of the lenticular lens to the display unit.
In contrast to this, the slit barrier is not fixed, but can be easily moved in the three-dimensional display unit of a liquid crystal parallax barrier system shown in FIG. 2. Accordingly, it is possible to cope with an arbitrary three-dimensional image display from the two-eye type to a multiple-eye type. The three-dimensional display unit can be also used as a normal two-dimensional image display unit in which no resolution is reduced. Further, it is possible to display two-dimensional and three-dimensional images on the same screen in a state in which these images ape mixed with each other.
However, there ape some faults in the three-dimensional display unit shown in FIG. 2. A first fault is a reduction in light amount caused by the slit barrier so that the screen becomes dark. A second fault is that the slit barrier becomes an eyesore obstacle at the observing time of an image. To avoid this second fault, it is necessary to set a pitch of slits of the slit barrier to be very small. However, when the slit barrier is constructed by a liquid crystal panel, the slit pitch is limited by a pixel pitch of the liquid crystal panel when the slit pitch of the slit barrier is reduced. Directivity of light is widened by a diffraction phenomenon even when a sufficiently small slit pitch is obtained. These two faults provide a limit of the parallax barrier system irrespective of a structure in which the slit barrier is constructed by a liquid crystal. Therefore, no parallax barrier system is considered as a practical technique at present so that no parallax barrier system is currently a main stream product current.
A third fault of the three-dimensional display unit shown in FIG. 2 is that the three-dimensional display unit is large-sized in comparison with a screen size. Further, a distance between the two liquid crystal panels must be increased as an observation distance is increased. Further, it is necessary to arrange a mechanical device for moving the liquid crystal panels forward and backward in accordance with a change in observation distance. A fourth fault of the three-dimensional display unit shown in FIG. 2 is that polarizing plates are required on both faces of each of the two liquid crystal panels used in the three-dimensional display unit. Accordingly, light from the back light is transmitted through a total of four polarizing plates. Therefore, an amount of the transmitted light is reduced since no transmittance of each of the polarizing plates is equal to 100%. A strong back light is required to compensate this reduced light amount. Accordingly, there are various kinds of problems about the general technique of the three-dimensional display unit.
FIG. 4 shows another general three-dimensional display unit using a lenticular lens. In this three-dimensional display unit, the lenticular lens is directly stuck onto the display face of a liquid crystal panel 121. This three-dimensional display unit is of a two-eye type in which two different parallax images are simultaneously displayed in the liquid crystal panel. One portion of a parallax image corresponding to a left-hand eye is displayed in a display pixel D.sub.i1 of the liquid crystal panel 121. One portion of a parallax image corresponding to a right-hand eye is displayed in a display pixel D.sub.i2 of the liquid crystal panel 121. A cylindrical lens L.sub.i is arranged such that this cylindrical lens corresponds to a pair of display pixels D.sub.i1 and D.sub.i2. Light is transmitted through the display pixels D.sub.i1 and D.sub.i2 and is separated into light portions in display spaces P and Q within an observation region by a converging operation of the cylindrical lens L.sub.i. Light is similarly separated into light portions with respect to index i from 1 to n. Thus, the parallax image for the left-hand eye is converged in the display space P and the parallax image for the right-hand eye is converged in the display space Q. A stereoscopic image can be observed when the left-hand and right-hand eyes are respectively located in the display spaces P and Q.
As mentioned above, in the three-dimensional display unit of a lenticular system, spaces capable of observing the stereoscopic image are limited and are spaced from each other.
A three-dimensional display unit of a multiple-eye type for regenerating many different parallax images is used in a certain case to widen spaces capable of observing a stereoscopic image. However, in this case, many different parallax images such as three parallax images or more must be simultaneously displayed in a liquid crystal panel 121. Accordingly, resolution of one parallax image is greatly reduced since the number of display pixels in the liquid crystal panel 121 is limited.
Therefore, another general three-dimensional display unit of a head tracing type is developed to observe a stereoscopic image having high resolution in a wider space. In this three-dimensional display unit, the position of an observer's head is detected while a stereoscopic image of the two-eye type is regenerated. A position of the regenerated stereoscopic image is conformed to the observer's head position.
For example, the observer's head position is photographed by a video camera at any time. The position of a contour of an observer's face or the position of an observer's eye is detected from an image signal of the video camera. An operation of the three-dimensional display unit is controlled such that the regenerated stereoscopic image is displayed in this detected position.
FIG. 5 is a view for explaining a basic principle of the three-dimensional display unit of the head tracing type. A lenticular lens 132 is arranged on the front face of a liquid crystal panel 131. The regenerating principle of a stereoscopic image is similar to that explained with reference to FIG. 4.
The differences between the regenerating principles shown in FIGS. 4 and 5 are that a lens moving device 133 is connected to the lenticular lens 132 so as to change a relative position of the lenticular lens 132 with respect to the liquid crystal panel 131. When the relative position of the lenticular lens 132 with respect to the liquid crystal panel 131 is changed, an emitting direction of light emitted from each of cylindrical lenses constituting the lenticular lens is changed by a converging action thereof so that display positions P' and Q' in display spaces can be controlled.
The lens moving device 133 is a device for exactly controlling a position of the lenticular lens 132. Accordingly, the lens moving device 133 is constructed by a precise mechanical system.
A regenerating position of the stereoscopic image is controlled by moving the lenticular lens 132 such that this regenerating position is in conformity with a detected observer's head position.
As mentioned above, in the general three-dimensional display unit of the two-eye type, a space capable of observing the stereoscopic image is greatly limited on the basis of the principle of the lenticular system.
In the general three-dimensional display unit of the multiple-eye type, the observation space of the stereoscopic image is widened in accordance with multiple eyes. However, resolution of one parallax image is reduced so that the quality of a regenerated stereoscopic image is reduced.
Further, in the general three-dimensional display unit of the head tracing type, relative positions of the liquid crystal panel and the lenticular lens must be very exactly controlled. Therefore, a precise mechanical system is used so that the three-dimensional display unit is large-sized. Accordingly, a relatively large lenticular lens is moved so that responsibility of position control in a display space is reduced. Further, the lenticular lens is moved only on a face parallel to the display panel so that a head tracing range is also limited on this face.
The general three-dimensional display unit of the head tracing type is of a two-eye type. Accordingly, no observed stereoscopic image is moved even when the observer's head is moved. Therefore, no natural stereoscopic image can be regenerated in this three-dimensional display unit.
In the three-dimensional display unit of a direct viewing type as another general example, a lenticular lens is directly stuck onto a liquid crystal panel display face.
One cylindrical lens corresponding to a plurality of pixels of the liquid crystal panel is prepared in the three-dimensional display unit of the direct viewing type. One portion of different parallax images is displayed in each of the plural pixels. Each of the parallax images is separately formed in a certain space in an observation region by a converging function of the cylindrical lens. An observer can observe a stereoscopic image if the observer sees the different parallax images by his right-hand and left-hand eyes.
When a pitch of the plural pixels of the above liquid crystal panel and a pitch of the cylindrical lens are equal to each other and all cylindrical lenses have the same shape, the size of an observable display screen is a small size about a distance between the observer's eyes.
To increase the size of the display screen, it is necessary to converge light emitted from a peripheral portion of the liquid crystal panel to the observation space prescribed by the distance between the observer's eyes. For example, a method for converging this light to the observation space is shown in FIG. 6.
FIG. 6 shows an example of the three-dimensional display unit of a two-eye type. In this three-dimensional display unit, one portion of a parallax image for a left-hand eye is displayed in a pixel G.sub.i1 of a liquid crystal panel 30. One portion of a parallax image for a right-hand eye is displayed in a pixel G.sub.i2 of the liquid crystal panel 30. Index i is set to a value from 1 to n. A cylindrical lens L.sub.i is arranged in accordance with a pair of pixels G.sub.i1 and G.sub.i2.
Light is transmitted through the pixels G.sub.i1 and G.sub.i2 and is separated into light portions in display spaces I and J in the observation region by a converging operation of the cylindrical lens L.sub.i. A stereoscopic image can be observed when the left-hand and right-hand eyes are respectively located in the display spaces I and J.
In FIG. 8, the cylindrical lens L.sub.i has the same shape. However, a pitch of the pair of pixels G.sub.i1 and G.sub.i2 is different from a pitch of the cylindrical lens L.sub.i. The pitch of the cylindrical lens L.sub.i is set to be slightly smaller than the pitch of the pair of pixels G.sub.i1 and G.sub.i2.
A center of the pair of pixels G.sub.in is shifted from a center of the corresponding cylindrical lens L.sub.i in a peripheral portion of the liquid crystal panel 30. An amount of this shift is increased as the shift is caused in the peripheral portion of the liquid crystal panel 30. Incident angles of transmitted light of the respective pixels G.sub.in incident to the cylindrical lens L.sub.i are different from each other by this shift in central and peripheral portions of the liquid crystal panel 30. Accordingly, the transmitted light from the pixels G.sub.in in the peripheral portion of the liquid crystal panel 30 can be converged into the specified spaces I and J in the observation region.
However, the above general three-dimensional display unit as a flat panel display has a wiring portion between pixels. No light is transmitted through this wiring portion. Accordingly, it is considered that black light is transmitted through this wiring portion. In this case, this black light is converged between the display spaces I and J in the specified observation region by a converging principle similar to the above-mentioned converging principle. This means that an unreachable region of the transmitted light from the pixels exists between the display spaces I and J.
FIG. 7 shows one example of the construction of a general flat panel display of a three-eye type.
In the flat panel display shown in FIG. 7, a cylindrical lens R.sub.i is arranged in proximity to display pixels Q.sub.i1, Q.sub.i2 and Q.sub.i3 of a liquid crystal panel 40 such that the cylindrical lens R.sub.i corresponds to these display pixels. The transmitted light is converged and formed as a parallax image in each of display spaces S, T and U within an observation region. No light is transmitted through a wiring portion between pixels of the liquid crystal panel so that this wiring portion forms a non-transmitting portion P.sub.i.
This non-transmitting portion P.sub.i causes a space to which no light is almost transmitted within the observation region. Namely, non-display spaces V and W are formed in accordance with non-transmitting portions P.sub.i1 and P.sub.i2.
An observer recognizes each of the non-display spaces V and W as a black band. The observer sees the black band as a non-display portion at any time when the observer moves his head and an observed stereoscopic image is changed from a combination of the display spaces S and T to a combination of the display spaces T and U.
FIG. 8 shows a light intensity distribution of a projecting pattern which is obtained by converging the transmitted light of each of the display pixels by the cylindrical lens R.sub.i and is taken along a cutting plane b-b' in FIG. 7.
In FIG. 8, the width of a non-display portion is widened in comparison with an ideal state. Accordingly, there is a problem that this widened non-display portion becomes a great obstacle when a continuous stereoscopic image is observed.
FIG. 9a is a plan view showing a liquid crystal panel 91. FIG. 9b is a cross-sectional view showing a lenticular lens 92 corresponding to the liquid crystal panel 91 shown in FIG. 9a.
FIGS. 9a and 9b show a two-eye type. With respect to the liquid crystal panel 91, a scanning operation is performed in a vertical direction such that a main scanning line is in conformity with the longitudinal direction of a cylindrical lens within the lenticular lens 92. The liquid crystal panel 91 displays images for right-hand and left-hand eyes in a mixing state every other main scanning line. Namely, the image for the right-hand eye is displayed on each of odd horizontal scanning lines such as (1), (3), (5), (7), - - - . The image fop the left-hand eye is displayed on each of even horizontal scanning lines such as (2), (4), (6), (8), - - - .
A projected image of the image for the left-hand eye shown in FIG. 10a is displayed by a converging action of the lenticular lens 92 stuck to a front face of the liquid crystal panel 91 in a certain space in an observation region. A projected image of the image for the right-hand eye shown in FIG. 10b is displayed in a spatial portion adjacent to this certain space.
When a direct current voltage is continuously applied to the liquid crystal panel, electrolysis of liquid crystal molecules is caused so that the liquid crystal panel is finally unoperated. Therefore, an alternating current voltage is normally applied to the liquid crystal panel. A system for applying the alternating current voltage to the liquid crystal panel is constructed by two systems composed of a line inverting system shown in FIG. 11a and a frame inverting system shown in FIG. 11b.
In the line inverting system shown in FIG. 11a, a voltage having positive and negative polarities is applied to the liquid crystal panel every one line (one horizontal period). In contrast to this, in the frame inverting system shown in FIG. 11b, a voltage having positive and negative polarities is applied to the liquid crystal panel every one frame (one vertical period).
As mentioned above, in the general voltage applying technique, the positive and negative polarities are inverted in the frame inverting system or the line inverting system. In the frame inverting system, the images for the left-hand and right-hand eyes respectively shown in FIGS. 10a and 10b and projected in the stereoscopic observation space are displayed by a positive or negative voltage having the same polarity. The images for the left-hand and right-hand eyes are displayed in the next frame by a negative or positive voltage having a polarity inverse to the previous polarity.
The images for the left-hand and right-hand eyes are instantaneously displayed by the same polarity at any time. However, in view of a change in time, the voltage polarities are repeatedly inverted every frame with respect to the images for the left-hand and right-hand eyes. A slight difference in voltage setting is caused by a difference between the voltage polarities so that a flicker phenomenon is caused. In this flicker phenomenon, light and dark portions are periodically repeated.
In the line inverting system, the image for the left-hand eye shown in FIG. 10a and projected in the stereoscopic observation space in a frame at a certain time is displayed by the same voltage polarity with respect to all lines. The image for the right-hand eye shown in FIG. 10b is also displayed by the same voltage polarity with respect to all lines. However, the voltage polarities with respect to the images for the left-hand and right-hand eyes are different from each other. The voltage polarities are inverted in the next frame with respect to each of the images for the left-hand and right-hand eyes. Therefore, the polarities of the applied voltage are also different from each other in the next frame with respect to the images for the left-hand and right-hand eyes.
Accordingly, the polarities of the applied voltage are different from each other in a frame at the same time with respect to the images for the left-hand and right-hand eyes so that a difference in brightness between the images for the left-hand and right-hand eyes is caused. Therefore, when an observer sees a stereoscopic image, images having different brightnesses are observed by the left-hand and right-hand eyes so that fatigue of the observer is increased.